1. No not any. Many cross-compilers do not if the binary formats and pecularities of arithmetic operations maps well between host system and target system - why waste time? Example = x86 and x64, two different platforms, with same binary representation. About performance - not sure if it is negligible - optimisation takes a lot of time in moder compilers. 3. You have not seen ml code I am sure. ML has type inference, you are not bothered about types too much. And it has pattern matching, very good when traversing trees. Neither scheme nor pyruby has this. ML also a language with completely formalized semantics, in combination with strong type it allows you to prove that you compiler is correct by means of formal provers.